1. Field of the Invention
This invention relates generally to a multi-component material having a wide range of compositions which is suitable for use in devices such as uncooled thermal imaging detectors. The invention also relates to a simple graphical means by which to provide a first order estimate of the desired composition of the multi-component material.
2. Discussion of the Prior Art
Thermal detectors and imagers which employ thermally sensitive materials may utilize ferroelectric and pyroelectric compounds such as barium strontium titanate. Each material has its characteristic Curie point, i.e., the transition temperature at which the material changes from being ferroelectric to paraelectric. This change is accompanied by a rapid change in the dielectric constant as the temperature reaches the Curie point. By selecting the proper amounts of related materials, a solid solution of the materials can be fabricated which has a room temperature (i.e., 25xc2x0 C.) Curie point. With such a material, very small changes in thermal energy can be detected as the material goes through the transition point, as manifested by large changes in either the electronic capacitance or the dielectric constant. These dramatic changes in values constitute an electronic signal. When this material is made into an array of very small detectors, the resulting combined electronic signals from all of the detectors produce an electronic image of the thermal scene.
Multi-component materials such as BaSrTiO3 have heretofore been employed in the imaging arrays of uncooled thermal detectors, which by design, operate at ambient or room temperature. Since BaSrTiO3 is, however, relatively expensive, other multi-component materials may be employed which possess similar characteristics and operate in the same manner.
A detector material consisting of PbSrTiO3 having a composition defined by the general formula Pb(1xe2x88x92x)SrxTiO3 is disclosed in U.S. Pat. No. 5,079,200, the disclosure of which is incorporated by reference herein. In one embodiment the material contains approximately 64-68% strontium titanate, which provides a detector material with a Curie point temperature and maximum sensitivity near standard or room temperature.
Thus, in order to reduce the expense associated with devices which employ BaSrTiO3, it is desirable to utilize alternative multi-component compositions which while operating in the same manner as BaSrTiO3, and also offer a broader range of potential compositions.
It is an object of the present invention to provide a multi-component material having a wide range of compositions which is suitable for use in devices such as uncooled thermal imaging detectors. It is a farther object of the invention to provide a graphical method which facilitates the preparation of a first order estimate of the composition of the multi-component material.
Accordingly, the present invention relates to a multi-component material represented by the general formula (I):
Ba(1xe2x88x92xxe2x88x92y)PbyMxTiO3xe2x80x83xe2x80x83(I)
in which:
M is Sr or Cd;
x is the decimal fraction molar concentration of MTiO3, where x has a value equal to or greater than about 0.20 and less than or equal to about 0.75;
y is the decimal fraction molar concentration of PbTiO3, where y has a value greater than zero and less than or equal to about 0.40; and
x+y is less than 1.0.
The invention also relates to a graphical method of estimating the composition of the multi-component material represented by the general formula (I) comprising the steps of:
selecting a value of x;
locating a first point on a first graphical representation comprising a temperature axis (y-axis) representing Curie points in xc2x0C. and a concentration axis (x-axis) representing mole percentage of BaTiO3 and MTiO3, the first point corresponding to the intersection of an abscissa representing x expressed as a mole percentage, with an ordinate representing a temperature (T) between about xe2x88x9240xc2x0 C. to 60xc2x0 C., with 25xc2x0 C. (room temperature) being preferred;
locating a second point on the first graphical representation corresponding to the intersection of an abscissa representing 100% MTiO3 with an ordinate representing a temperature of xe2x88x92220xc2x0 C.;
drawing a straight line from the second point through the first point, and extending said line to intercept the temperature axis and line DE at a third point; where line DE now represents the decimal mole fraction of PbTiO3 in the BaTiO3/PbTiO3 system (See FIG. 1-B);
and multiplying the decimal mole fraction of PbTiO3 identified by the third point by a value equal to 1xe2x88x92x to determine the value of y.
Thus, by selecting a value of x within the allowed range for a desired temperature (preferably ambient or room temperature) operation, and then employing graphical means, the corresponding value of y can be determined, and a first order estimate of the desired composition of a 3-component titanate can be provided.